Sin probar
body{
    background-color: wheat;
}

/*cabecera*/
.header{
    font-size: 4em;
    text-align: center;    
}

/*Menu*/
.menu{
    height: 2em;
}

.menu li{
    background-color: #F08080;
    border-radius: 2em;
    display: inline;
    float: left;
    height: 3em;
    list-style: none outside none;
    margin-right: 5%;
    width: 20%;   
}

.menu li a{
    color: #FFF5EE;
    display: block;
    font-size: 1em;
    padding: 0.45em 1em 1em;
    text-align: center;
    text-decoration: none;
}

h2.agenda{
    font-weight: bold;
    margin-left: 12.5%;
    margin-top: 2.5em;
}

h3{
    margin-top:3em;
    margin-left:25%;
}

/*Tabla*/
table{
    margin: auto;
    background-color: white;
    border: solid 1px;
    border-collapse: collapse;
    width: 50%;
    text-align: center;
}

th{
    background-color: tomato;
    font-weight: bold;
    border-bottom: solid 2px;
}

.tabla-centrada{
    width: 75%;
}

.tabla-centrada td{
}

.tabla-centrada th{
    background-color: lightgrey;
    font-weight: bold;
    border-bottom: solid 2px;
}

/*Boton volver a principal*/
.volver{
    margin-top: 3em;
    float: right;
    border-radius: 2em;    
    background-color: blueviolet;
    display: block;
    font-size: 1.5em;
    padding: 0.45em 1em 1em;
    text-align: center;
    text-decoration: none;    
}

.volver a{
    color: white;
}